It is known how to manufacture from thermoplastic resins, such as polyvinyl chloride (PVC) and acrylonitrile-butadiene-styrene copolymers (ABS), finished articles presenting improved mechanical characteristics by orienting the macromolecules. Such is now being practiced in the domain of synthetic fibers. Industrial applications of this principle are starting to be developed in order to obtain plates, films, tubes or bottles, particularly in PVC. Orientation can take place in one direction or in two orthogonal directions, and in proportion to the orientation achieved, we can observe an important increase in the rigidity and in the impact strength (resistance to shock), whereas the gas permeability diminishes. This orientation operation carried out in continuous or in discontinuous manner comes to be added to the customary stages of the shaping (molding) operation of the thermoplastic polymers. The practical conditions for obtaining a substantial orientation of the macromolecules, for instance the temperature at which the operation must take place, are not customary and pose problems with regard to the production of the material.
In order to orient the macromolecules to the maximum and to preserve a sufficient level of orientation in the finished material, a high speed traction must be exerted on the material, and this while the polymer is in its visco-elastic phase, that is to say at an unusually low temperature, near the glass transition temperature; i.e., for example, for PVC in the 85.degree.-110.degree. C. interval. Then the stretched material must be cooled rapidly, as soon as the end of drawing, in order to avoid the relaxation phenomenon, if one desires to preserve the maximum benefit of the achieved orientation. It has been observed that the closer the temperature of the material is, at the moment it is being stretched, to the glass transition temperature, the more effective is the final orientation or residual orientation or orientation which subsists in the material and the better is the rigidity of the final product after cooling.
On the other hand, the closer the vitreous transition temperature is being approached, the higher is the stress to be applied onto the material in order to stretch it to orient it. Consequently, the means required for the orientation of a flat plate, for instance, which comprises fastening devices attached to the plate on its edges and one or two traction mechanisms, will be all the more voluminous and costly when it is desired to stretched the material at low temperature. In the same manner, if one desires to orient a tube in both directions, the longitudinal traction and the internal pressure to be exerted will be all the greater, thus more costly, with lower temperatures being approached.
A priori it would thus be advantageous to stretch the material at a temperature clearly higher than the glass transition temperature, for instance in the 130.degree.-150.degree. C. zone, since the necessary stress would be lower. Unfortunately, at these higher temperatures, the elongation capacity of the material is such smaller and the relaxation velocity of the polymer chains is higher, so that only elongations insufficient to provide an orientation which appreciably improves the final mechanical characteristics can be obtained. Under these conditions, only an elongation of the order of 100% can be obtained for the PVC, for instance, and such an elongation of the order of 100% is not sufficient for bringing on a residual or final orientation.
These consideration on the behavior on stretching of a thermoplastic resin in its elastic phase, as a function of the temperature, are illustrated by FIG. 1, a graph, which at (a) gives the elongation at break, at (b) the stress at break and at (c) the stress at 200% of elongation as a function of the temperature, for non-plasticized PVC stretched at a speed of 666% per minute. FIG. 2 is a graph showing the stress at break in traction at 23.degree. C. and at a speed of 5 mm/minute of non-plasticized PVC specimens previously oriented by 200% elongation at a speed of 666%/min; then rapidly cooled down, as a function of the temperature at orientation. By considering these two graphs simultaneously, it can be seen that if one wishes to stretch by 200%, a better final rigidity is obtained by doing it at 100% C. rather than at 120.degree. C., but then the stress force to be exerted amounts to 3.5 MPa instead of 2.5 MPa.